The present disclosure relates to surfaces that exhibit superhydrophobic properties when treated with a composition including a water-based, non-organic solvent.
A superhydrophobic surface exhibits a sessile water contact angle of greater than 150°. If, additionally, the surface exhibits a water droplet roll-off (sliding) angle of less than 10°, the surface is deemed to be “self-cleaning.” In nature, lotus leaves exhibit such properties (so-called lotus effect). Most man-made materials such as fabrics, nonwovens, cellulose tissues, polymer films, etc., do not have surfaces with such properties. Currently, there are generally two methods to modify a non-superhydrophobic surface to achieve the lotus effect. One method is to graft a hydrophobic monomer onto every surface of a non-superhydrophobic material. Such a method makes the material superhydrophobic throughout the thickness of the material, which might not be desired in most cases. It is also not cost effective, cannot be used for a continuous production, and can lead to undesirable environment issues.
Another approach is to coat a specially-formulated liquid dispersion onto a surface. Upon subsequent drying, a nano-structured superhydrophobic film forms. To use such an approach, the deposited film must exhibit a chemical and physical morphology characteristic of superhydrophobic surfaces. First, the formulation requires at least one low-surface energy (i.e., hydrophobic) component, and second, the treated surface has to have a rough surface texture, preferably at several length-scales: micro- and nano-roughness. Although various formulated dispersions capable of achieving a superhydrophobic surface exist, none of these dispersions are purely water-based.
Low-cost, large-area superhydrophobic coating treatments are of great value to many applications requiring a passive means for attaining efficient liquid repellency. While many applications are envisioned, only few are realizable due to either the high-cost or low-durability of such treatments. Recently, spray deposition of polymer-particle dispersions has been demonstrated as an excellent means for producing low-cost, large-area, durable, superhydrophobic composite coatings/films; however, the dispersions used for spray deposition of superhydrophobic coatings generally contain harsh or volatile solvents. Solvents are required for solution processing of polymers as well as for dispersing hydrophobic nanoparticles, thus inhibiting scalability due to the increased cost in chemical handling and safety concerns. This problem can be overcome by replacing solvents with water, but this situation is paradoxical: producing a highly water-repellent coating from an aqueous dispersion.
Also, such coatings usually contain fluoropolymers. A low-surface energy polymer (˜20 mN/m) must be incorporated into the coating (a general requirement of any liquid repellent surface) which is conveniently achieved by utilizing fluoropolymers (e.g., fluoroacrylic copolymers, poly(tetrafluoroethylene), etc.). However, concerns over their bio-persistence have provided an impetus for eliminating these chemicals. The problems with the byproducts of fluoropolymer degradation, e.g. long-chain perfluorinated acids (PFAs), which have a documented ability to bioaccumulate, as well as the potential adverse effects PFA in maternal concentrations can have on human offspring, have led to a shift in the manufacture and usage of fluoropolymers. One common PFA of particular concern is perfluorooctanoic acid (PFOA). In 2006, the EPA introduced its PFOA (perfluorooctanoic acid) Stewardship Program and invited eight major fluoropolymer and telomer manufacturers to commit to eliminating precursor chemicals that can break down into PFOA; in one case, DuPont introduced so-called short-chain chemistry, whereby the length of perfluorinated chains within polymers are kept below a threshold in order to avoid degradation into PFOA. In other applications, usage of fluoropolymers in products that come in sustained contact with the human body or in disposable items intended for landfilling after consumption must be minimized.
Approaches to utilizing graphene/graphite in superhydrophobic applications are reported in the literature and a few will be briefly discussed here to demonstrate its applicability. In a recent report, a dispersion consisting of colloidal graphite and polytetrafluoroethylene was spray cast and sintered to form the basis for a conductive, thermally stable, water-repellent coating. Other approaches to utilizing graphite—or its exfoliated form, graphene—to form superhydrophobic films have included aerogels, poly(vinylidene fluoride) composites, and Nafion blends. Graphene oxide films can also be superhydrophobic when modified by octadecylamine; however, this is not suitable for many applications for the same reason organoclays are not being used. Work regarding wettability tuning for graphene films to water has been done, but it relies on chemisorption of acetone to defects in graphene reduced from graphene-oxide as its mechanism. In all of these studies, none of the systems were water-based, and many contained some type of fluoropolymer, which makes them not environmentally friendly or benign.